This invention relates to the detection of moving objects, and, more particularly, to a technique suitable for use by automated sensor systems.
The ability to detect small moving objects in an automated system is important in a variety of contexts. For example, most fighter aircraft utilize air-to-air missiles as their primary offensive weapon. To minimize the danger to the launch aircraft, the missile is desirably launched when the target is at a great distance from the aircraft and directed to the target. The missile usually has a sensor that is useful when the target is nearly directly forward of the missile and fairly close to the missile, and is thus of most value for terminal guidance.
In a typical scenario, a target is first acquired by a targeting aid such as a ground-based radar or an AWACS aircraft. After the target is identified as potentially hostile, this information and the approximate location and velocity of the target are communicated to the launch aircraft. The targeting information is downloaded into the missile guidance computer, and the missile is launched. The missile then flies a pattern either directly toward the target on in a circuitous path so that the target continues to exhibit relative motion. The position and velocity of the target may be periodically updated with messages from the targeting aid to the missile guidance computer in some designs, and there may be no communication with the missile after launch in other designs. For a successful mission, the sensor of the missile acquires the target during the terminal flight phase and guides the missile to the target.
In the terminal phase of the flight, the missile targeting computer must be able to identify the target. In an ideal situation, the target would be the only feature in the field of view of the sensor and would have a high contrast level against a uniform background such as the sky. The ideal situation is seldom realized, and instead in the usual case the missile targeting computer must be able to automatically identify a moving target in the midst of background clutter and other features in the field of view of the sensor. The detection of a small moving object becomes more difficult when the background is cluttered by other objects such as reflections from the surface of the sea or the great variety of features that are seen when an object moves with the earth or the horizon as the background. In these cases, the potential target is most likely smaller and has lower contrast relative to the majority of the background clutter.
Several techniques have been developed to address this complex problem. In one, spatial filters are used to remove objects larger than a preselected size from a scene. In many cases, however, the object is about the same size as much of the clutter, and therefore the clutter cannot be removed using only a spatial filter. In another technique, the stationary backgrounds are aligned in successive images and subtracted, leaving only moving objects (including the target) for subsequent threshold detection. While operable for many situations, this approach has the limitation that there must be an accurate registration of successive images. In another technique, the power spectrum of the scene is used to suppress the background clutter. This approach requires a priori knowledge of the power spectrum unique to the particular clutter background or the ability to determine that power spectrum rapidly for each scene. Further, many small clutter objects contribute little to the overall power spectrum of the scene, and therefore may not be effectively suppressed by the power spectrum approach. All of these approaches have difficulty in suppressing line-like features that extend parallel to the direction of movement of the target.
There is therefore a need for an improved approach for detecting small moving objects against a cluttered background and in the practical context required for military systems. The present invention fulfills this need, and further provides related advantages.